The optical signal-to-noise ratio (OSNR) is a direct measure of the quality of signal carried by an optical telecommunications link. The OSNR is the ratio of the power of the signal in the channel to the power of the noise in the channel:
                    OSNR        =                              P            signal                                P            noise                                              (                  Eq          .                                          ⁢          1                )            The OSNR may provide a very good estimate of the bit-error rate (BER) of the optical link and, as such, a measurement of the OSNR is often important for real-time optical network management at the physical layer.
The OSNR of an optical communications link is typically high, often in excess of 15 dB, 20 dB, or greater, under normal and proper operating conditions. The dominant component of the noise in the link is typically the result of a background effect known as amplified spontaneous emission (ASE). ASE is a broadband noise source contributed by the optical amplifiers in the link.
The limiting noise source in most long-haul optical networks is the signal-ASE beat noise, in which the signal and the ASE coherently mix. In typical optical communications systems using optical amplifiers, signal-ASE beat noise is the limiting noise term for optical performance. Thus, direct measurements of the ASE level and the signal level provide an indicative measure of the OSNR of the system and Eq. 1 becomes:
                    OSNR        =                              P            signal                                P            ASE                                              (                  Eq          .                                          ⁢          2                )            The noise power in this calculation is typically normalized to represent the power in a Reference Bandwidth (RBW), typically 0.1 nm. Since the OSNR approximates the BER of the channel, direct measurements of the ASE level and the signal level may ultimately provide the BER of the channel. A methodology that provides a direct and accurate measurement of the ASE level is therefore useful for high speed optical network monitoring.
Referring to FIG. 1, a dense wavelength division multiplexing (DWDM) optical system carries a number of distinct optical channels utilizing different optical frequencies. The available optical channels in the system are typically evenly spaced in frequency on the so-called ITU (International Telecommunications Union) grid. Two adjacent utilized channels therefore have closely-spaced frequency spectra, as shown in FIG. 1.
One method for measuring the level of the ASE noise in an optical link is to make a power measurement at a frequency between two adjacent DWDM channels (i.e., an out-of-band measurement) and to assume that whatever power is measured at this frequency is due solely to ASE effects. This assumes that the “signal” midway between the center frequencies of two adjacent DWDM channels is primarily due to ASE noise.
This assumption is valid to the extent that the ASE has a “white” optical spectrum, thereby being substantially uniform across various wavelengths (or frequencies). However, the optical components in a DWDM system, e.g., optical bandpass filters and other component filter functions, typically spectrally shape the ASE spectrum so that it varies among particular measurement frequencies.
Moreover, the signal spectrum of the channel has a spectral shape that is itself influenced by the modulation format of the data carried by the channel, and is similarly altered by the filter functions of the aforementioned elements of the system. For example, RZ data modulation formats (or many of the variations thereof) have wide modulation bandwidths. The data formats have power levels at the frequencies midway between adjacent ITU center channel frequencies that are sufficient to distort attempted measurements of ASE noise at those out-of-band frequencies. That is, with such wide modulation bandwidths, the signal spectra from adjacent DWDM channels may overlap or cross at the midpoint between adjacent channels with power levels comparable to or in excess of the power level of the ASE noise.
The result is that the signal spectra masks the underlying ASE noise level even when the ASE measurement is performed reasonably far away from the center of a given DWDM channel, thereby decreasing the available dynamic range or sensitivity of the OSNR measurement. The net result is that out-of-band noise measurements—made at the mid-point between adjacent DWDM channels—are typically not sufficiently indicative of the in-band noise levels which actually impact system performance.
A second problem with this methodology is that adjacent DWDM channels may originate at different locations and thereby accumulate different amounts of ASE noise en route to the location of an optical monitor. A measurement at or near the mid-point frequency between adjacent channels (as described above) may then measure a combination of the ASE noise levels associated with the two adjacent DWDM channels. Therefore, it would be advantageous to be capable of performing in-band ASE measurements at wavelengths not restricted to the band including the mid-point frequency between adjacent DWM channels.